In osmotic delivery systems such as described in U.S. Pat. No. 5,728,396 and U.S. Pat. No. 6,524,305, a piston is positioned in the lumen of a reservoir to divide the lumen of the reservoir into two chambers. The first chamber contains an osmotic agent formulation while the second chamber contains an active agent formulation. The piston isolates the osmotic agent formulation from the active agent formulation by engaging and sealing against the wall of the reservoir. Pressure differential across the piston allows the piston to move longitudinally within the reservoir. The piston is generally required to maintain its seal with the wall of the reservoir as it moves within the reservoir. The piston is typically made of a material that is of lower hardness than the reservoir, that will deform to fit the lumen of the reservoir, and that is impermeable. Typically, the piston is made of an elastomeric material, examples of which include, but are not limited to, the following: polypropylene; rubbers such as ethyl propylene diene rubber, silicone rubber, butyl rubber, chlorinated rubber, styrene-butadiene rubber, or chloroprene rubber; and thermoplastic elastomers such as plasticized polyvinylchloride, polyurethane, SANTOPRENE® (Advanced Elastomer Systems, Akron Ohio), or C-FLEX® (Consolidated Polymer Technologies, Inc., Clearwater Fla.).
There continues to be a desire to improve compatibility and sealing of pistons with components of the osmotic delivery systems.